Wavefield extrapolation modeling for internal multiple prediction

ABSTRACT

Methods for attenuating multiple reflections in seismic data by predicting the multiples using wavefield extrapolation modeling, which uses one-way wavefield propagation in both the up and down directions to predict internal multiples up to a specified order.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Application Ser. No. 61/119,640 filed on Dec. 3, 2008, withthe same title and by the same inventors (Attorney Docket No.594-25661-PRO).

BACKGROUND

1. Field of the Invention

Embodiments of the present invention generally relate to seismicsurveying and, more particularly, to a method for attenuating multiplesin seismic data.

2. Description of the Related Art

The following descriptions and examples do not constitute an admissionas prior art by virtue of their inclusion within this section.

Seismic surveying is a method for determining the structure ofsubterranean formations in the earth. Seismic surveying may typicallyutilize seismic energy sources which generate seismic waves and seismicreceivers which detect seismic waves. The seismic waves may propagateinto the formations in the earth, where a portion of the waves mayreflect from interfaces between subterranean formations. The seismicreceivers may detect the reflected seismic waves and convert thereflected waves into representative electrical data. The seismic datamay be transmitted by electrical, optical, radio or other means todevices which record the data. Through analysis of the recorded seismicdata (or seismograms), the shape, position and composition of thesubterranean formations may be determined. Such analysis may indicatethe presence or absence of probable locations of hydrocarbon deposits orother valuable substances.

Depending on the location where a survey takes place, there are surveysin sea, on land or in transition zones. Marine seismic surveying is amethod for determining the structure of subterranean formationsunderlying bodies of water. Marine seismic surveying may typicallyutilize seismic energy sources and seismic receivers located in thewater which may be either towed behind a vessel or positioned on thewater bottom from a vessel. The energy source may typically be anexplosive device or compressed air system which generates seismicenergy, which then propagates as seismic waves through the body of waterand into the earth formations below the bottom of the water. As theseismic waves strike interfaces between subterranean formations, aportion of the seismic waves may reflect back through the earth andwater to the seismic receivers, to be detected, transmitted, andrecorded. The seismic receivers typically used in marine seismicsurveying may be pressure sensors, such as hydrophones. Additionally,motion sensors, such as accelerometers, may be used. Both the sourcesand receivers may be strategically repositioned to cover the surveyarea.

Land seismic surveying is done on land. The energy sources are typicallyvibratory sources (vibrators). The vibrators produce a pressure signalthat propagates through the earth into the various subsurface layers.Here elastic waves are formed through interaction with the geologicstructure in the subsurface layers. Elastic waves are characterized by achange in local stress in the subsurface layers and a particledisplacement, which is essentially in the same plane as the wavefront.Acoustic and elastic waves are also known as pressure and shear waves.Acoustic and elastic waves are collectively referred to as the seismicwavefield.

A reflected wavefield may consist of both primary reflections andmultiple reflections. Primary reflections may be defined as seismicwaves which have reflected only once, from an interface betweensubterranean formations, before being detected by a seismic receiver.Primary reflections contain the desired information about thesubterranean formations which is the goal of marine seismic surveying.Multiple reflections, or multiples, may be defined as seismic waveswhich have reflected more than once before being detected by a seismicreceiver.

FIG. 1 illustrates two types of multiples: internal multiple 110 and asurface multiple 120. A “surface multiple” is herein defined as anyseismic event that is generated by at least two upward reflections andat least one downward reflection from the free surface boundary. Thefree surface boundary is typically the sea surface in marineenvironment, or earth surface in land surveys.

Another type of multiples is an “internal multiple”, which is hereindefined as any seismic event that is generated by at least two upwardreflections and at least one downward reflection from a boundary belowthe free surface with no downward reflection from the free surface. Theinternal multiples comprise multiple reflections between reflectors andmedia within the earth subsurface, whose physically properties areunknown and usually need to be determined by the survey.

In the context of seismic surveying, multiple attenuation is a pre-stackinversion of a recorded wavefield, which aims at removing the energyassociated with multiple reflections. Theoretically, multipleattenuation can be pursued in a totally data-driven manner by evaluatingequations which involve continuous summations over the directions ofspace.

Surface Multiples

There are several methods available to attenuate surface relatedmultiples. Wavefield extrapolation techniques that estimate surfacemultiples have been described in many publications. Wiggins (1999)summarized the 2D technique that subtracts forward extrapolated shotgathers from backward extrapolated shot gathers. In this case, theprimaries of the forward extrapolated shots matched the first ordersurface multiples of the backward extrapolated shots. Subtracting thesedata sets and forward extrapolating the results removed the first ordersurface multiples from the input shot gathers.

Kabir et. al. (2004) extended Wiggins' method to 3D and used it toattenuate water-bottom multiples and peg-legs. But this method requiredreceiver line interpolation to ensure adequate crossline data coverage.

Pica et. al. (2005) extended the 3D wavefield extrapolation technique toessentially de-migrate a depth image using a given velocity field tocompute primaries and surface multiples. The first de-migration processresulted in an estimate of the primaries, and subsequent de-migrationscomputed successively higher orders of surface multiples. They indicatedthat a recorded shot can be used in place of the computed primarywavefield.

Stork et. al. (2006) used a similar de-migration scheme to computeprimaries and surface multiples. This method is called WavefieldExtrapolation Multiple Modeling (WEMM), and performs the following foursteps. FIGS. 6-9 illustrate the steps and a four-layer model 600, whichhas three interfaces 611, 612 and 613, and a bottom 614.

Step 1: Using wavefield extrapolation, forward propagate a point source620 at the source location (S, considered the surface) downward throughthe given reflectivity and velocity models 600, and compute and storeupward reflection information (a 628, b 627 and c 626 in FIG. 6). Stopthe propagation at a specified depth (bottom 614).

Step 2: Forward propagate a 2D zero energy wavefield from the bottom 614of the given reflectivity and velocity models 600 to the surface 610,and accumulate all upward reflection information previously computed (c′636, b′ 637 and a′ 638 in FIG. 7). Save the 2D wavefield at the surface610. This wavefield consists of primary reflections generated from oneupward bounce off of the modeled reflectors 611, 612 and 613.

Step 3: Forward propagate the recorded 2D wavefield at the surfacedownward through the given reflectivity and velocity models 600, andagain compute and store upward reflection information (d 648, e 647 andf 646 in FIG. 8). Stop the propagation at the bottom 614.

Step 4: Forward propagate a 2D zero energy wavefield from the bottom 614of the given reflectivity and velocity models 600 to the surface 610,and accumulate all upward reflection information previously computed (d′658, e′ 657 and f′ 656 in FIG. 9). Save the wavefield at the surface610. This wavefield consists of first order surface multiples generatedfrom two upward reflections off of the modeled reflectors and onedownward bounce off of the free surface 610.

Higher orders of surface multiples can be computed by increasing thenumber of propagations performed. Surface multiples of order 1 to N−1are computed from N propagations.

Internal Multiple Prediction

Pica and Delmas (2008) used a wavefield extrapolation method similar tothe surface multiple prediction method described by Pica et. al. (2005)to compute 3D internal multiples from marine data. Their method consistsof four separate extrapolation steps.

Step 1: Back propagate recorded shots to an arbitrary depth.

Step 2: Upward propagate the back propagated shot through the migratedimage to generate a first set of secondary sources at reflectors.

Step 3: Propagate the wavefield generated from first set of secondarysources downward through migrated image, building a second set ofsecondary sources at the reflectors.

Step 4: Propagate the wavefield generated from the second set ofsecondary sources upward through migrated image, resulting in aninternal multiple model.

This method of internal multiple modeling inputs recorded shots as theinitial wavefield, and adds to all events in the shot of first orderinternal multiple whose generating horizon lies above the arbitrarydepth established in Step 1.

As described above, the prior art methods have some success inpredicting and attenuating multiples. But it is still not satisfactoryfor many situations. Internal multiples are still difficult and costlyto remove from seismic data, especially as the dimension for the problemincreases. It is desirable to have methods to effectively attenuatevarious multiples, especially internal multiples.

BIBLIOGRAPHY

-   Kabir, N., R. Abma and G Xia, 2004, 3D Wavefield Extrapolation based    demultiple in Ormen Lange, 74^(th) Annual International Meeting:    SEG, Expanded Abstracts, 1245-1248.-   Pica, A, G. Poulain, B. David, M. Megesan, S. Baldock, T.    Weisser, P. Hugonnet and P. Herrmann, 2005, 3D surface-related    multiple modeling, The Leading Edge, 292-296.-   Pica, A. and L. Delmas, 2008, Wave equation based internal multiple    modeling in 3D, 80^(th) Annual International Meeting: SEG, Expanded    Abstracts, 2476-2480.-   Stork, C., J. Kapoor, W. Zhao, B. Dragoset and K. Dingwall, 2006,    Predicting and removing complex 3D surface multiples with WEM    modeling—an alternative to 3D SRME for wide azimuth surveys, 76^(th)    Annual International Meeting: SEG, Expanded Abstracts, 2679-2683.-   Wiggins, W., 1999, Multiple attenuation by explicit wave    extrapolation to an interpreted horizon, The Leading Edge, 46-54.

SUMMARY

Described herein are implementations of various techniques for a methodfor attenuating internal multiple reflections using wavefieldextrapolation modeling. In one embodiment, the method starts with amulti-layer earth model, then carries out several steps, including: (a)propagating a wavefield downward until reaching the bottom of the earthmodel, computing and preserving information about any upward reflectingenergies at each layer boundary; (b) propagating the wavefield upwarduntil reaching the surface, computing and preserving information aboutany downward reflecting energies at each layer boundary; and (c)repeating step (a) and (b) at least once and accumulating upwardenergies towards a receiver as a prediction of multiples. Otherembodiments may include other steps.

The above referenced summary section is provided to introduce aselection of concepts in a simplified form that are further describedbelow in the detailed description section. The summary is not intendedto identify key features or essential features of the claimed subjectmatter, nor is it intended to be used to limit the scope of the claimedsubject matter. Furthermore, the claimed subject matter is not limitedto implementations that solve any or all disadvantages noted in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of various techniques will hereafter be described withreference to the accompanying drawings. It should be understood,however, that the accompanying drawings illustrate only the variousimplementations described herein and are not meant to limit the scope ofvarious techniques described herein.

FIG. 1 illustrates a diagrammatic view of an internal multiple and asurface multiple.

FIG. 2 illustrates center lines of a simple 3D reflectivity modelconsisting of 3 events within a constant velocity field.

FIG. 3 illustrates primaries computed by an embodiment of the currentinvention (WEM IMP) by propagating a wavefield through the reflectivitymodel and velocity model from FIG. 2 (first down/up propagation).

FIG. 4 illustrates first order internal multiples computed by a WEM IMPmethod by propagating the estimated primary wavefield through thereflectivity and velocity models (second down/up propagation). Theraypaths of these multiples are shown in FIG. 10.

FIG. 5 illustrates second order internal multiples computed by a WEM IMPmethod by propagating the estimated first order internal multiplewavefield through the reflectivity and velocity models (third down/uppropagation).

FIGS. 6-9 illustrate steps in a prior art method called WavefieldExtrapolation Multiples Modeling (WEMM). FIG. 6 shows a first downwardpropagation of point source. FIG. 7 shows a first upward propagation ofzero wavefield plane. The wavefield at the surface consists of primariesonly. FIG. 8 shows a second downward propagation of the 2D wavefield atthe surface. FIG. 9 shows a second upward propagation of zeroedwavefield plane. The wavefield at the surface (R) consists of firstorder surface multiples only.

FIG. 10 illustrates raypaths of all possible internal multiples for asimple 4 layer synthetic which is used as a model to illustrate anembodiment of the current invention.

FIG. 11 illustrate a first step of an embodiment. A wavefield isgenerated from a point source at S, and propagated through the earthmodel as shown in FIG. 10. During the first downward propagation, theupward reflecting energy (a, b and c) are computed and recorded.

FIG. 12 illustrates a second step of the embodiment. At the bottom ofthe downward propagation, the wavefield is zeroed. During the firstupward propagation, the previously recorded upward reflections (a, b andc) are added to the upward propagating wavefield (c′, b′ and a′), andthe downward reflecting energy is computed and recorded (d and e). Thefinal wavefield recorded at the surface contains only primaryreflections.

FIG. 13 illustrates a third step of the embodiment. At the surface, thewavefield is zeroed. During the second downward propagation, the upwardreflecting energy is recorded (f and g) and the previously recordeddownward reflections (e′ and d′) are added to the downward propagatingwavefield

FIG. 14 illustrates a fourth step of the embodiment. For the secondupward propagation, the wavefield energy at the bottom is again zeroed,and previously recorded upward reflections (f and g) are added to theupward propagating wavefield (f′ and g′). The wavefield recorded at thesurface receiver location (R) contains only first order internalmultiples.

FIG. 15 illustrates an additional step of the embodiment. Any number (N)of propagations can be run to produce internal multiples of orders 1 toN−1.

FIG. 16 illustrates another optional step. If the wavefield is notzeroed at surface, both surface multiples and internal multiples arecomputed.

FIG. 17 illustrates combinations of the primary, surface multiples andinternal multiples.

FIG. 18 illustrates a flow chart of an embodiment.

FIG. 19 illustrates a computing system, into which implementations ofvarious techniques described herein may be implemented.

DETAILED DESCRIPTION

The discussion below is directed to certain specific implementations. Itis to be understood that the discussion below is only for the purpose ofenabling a person with ordinary skill in the art to make and use anysubject matter defined now or later by the patent “claims” found in anyissued patent herein.

The following paragraphs generally describe one or more implementationsof various techniques directed to a method for predicting multiples,especially internal multiples and use the predicted multiples toeventually attenuate such multiples from seismic data.

One embodiment of the current invention is Wavefield ExtrapolationModeling for Internal Multiple Prediction (WEM IMP), which is amodel-based method that uses one-way wavefield propagation in both theup and down directions to predict internal multiples up to a specifiedorder. The WEM IMP algorithm is a modification of an existing WavefieldExtrapolation Multiple Modeling (WEMM) algorithm that computes surfacemultiples. The WEM IMP method can also compute surface multiples andcombinations of surface and internal multiples.

The WEM IMP algorithm uses a reflectivity model and a velocity model. Itdoes not require any input data. A hypothetical source wavefield ispropagated into the earth model. The velocity model determines thevelocity of the waves and the reflectivity model determines where thereflections occur and how much energy is reflected. The reflected energyis ultimately propagated back to the surface where it is recorded asseismic data. FIG. 2 shows a simple constant velocity reflectivitymodel. It corresponds to a model as shown in FIG. 10. WEM IMP propagatesenergy through the reflectivity and velocity models 1000 to computeestimates of the primaries as illustrated in FIG. 3. Additionalpropagations of the resulting primary wavefield yield increasing ordersof multiples as illustrated in FIGS. 4 and 5.

FIG. 10 shows a simple four layer geologic model 1000 and all possiblefirst order internal multiples that can occur. All of these multiplesare computed by WEM IMP without the need to know which horizonsgenerated the internal multiple (i.e. produced the downwardreflections).

The reflections being produced by multiplying the wavefield at a certainspatial location by the reflectivity at the same spatial location can beobtained from the reflectivity model (e.g. the cubes). The reflectivitymodel may be expressed in a collection of an x-y-z cube of grid points.Where reflections occur, the values at the grid points will be non-zero.At locations where there are no reflectors the values will be zero. Soonce the reflectivity cube is given in the model, the locations ofindividual reflectors are immaterial.

Each of the multiples depicted in FIG. 10: 1002, 1004, 1006, 1008, 1012(a, b, c, d and e) are predicted by WEM IMP in FIG. 4: 402, 404, 408 and412 (a, b+c, d and e). The raypaths of 1004 b and 1006 c have similararrival times.

Higher order multiples can be similarly predicted using the WEM IMPmethod. FIG. 5 illustrates a result including the higher ordermultiples.

Similar to WEMM, WEM IMP may have four basic steps. WEM IMP may alsohave several optional steps. WEM IMP performs a series of downward andupward propagations through the earth model. The following four stepsare performed in computing the first order internal multiples.

Basic Steps

Step 1: Energy from a point source located at the source position S(FIG. 11) is downward propagated through the earth model 1000. As energyis propagated downward (1121, 1122, 1123 and 1124), upward reflectingenergy (1126, 1127 and 1128) is computed at each reflection boundary(1021, 1022 and 1023) as illustrated in FIG. 11. These calculatedenergies are saved. This step is similar to the first step in WEMM.

Step 2: Once the maximum depth 1024 is reached, a 2D wavefield with zeroinitial energy is propagated from the bottom of the model upward. Upwardreflecting energies 1236, 1237 and 1238 as shown in FIG. 12 are added tothe wavefield as it is propagated upward through the earth model 1000.The wavefield recorded at the surface then contains only primary energythat has been reflected upwards.

As the wavefield is being propagated upward, downward reflectingenergies 1257 and 1258 are computed at each reflection boundary 1022 and1021 in FIG. 12. They are saved. Starting from this step, WEM IMPdiffers from WEMM. Since the wavefield started with zero energy, alldownward reflected energies 1257 and 1258 have originated from theaccumulation of energies from the upward reflections 1236, 1237 and 1238as shown in FIG. 12.

At the bottom of the downward propagation, the wavefield is zeroed.During the first upward propagation, the previously recorded upwardreflections 1126, 1127 and 1128 (a, b and c) are added to the upwardpropagating wavefield 1236, 1237 and 1238 (c′, b′ and a′), and thedownward reflecting energy is computed and recorded (d and e). The finalplanar wavefield recorded at the surface contains only primaryreflections.

Step 3: A 2D planar wavefield at the surface is propagated downwardbeginning with zero energy. Downward reflecting energies 1322 and 1323(e′ and d′ in FIG. 13) are added to the wavefield as they are propagateddownward through the earth model 1000. This accumulation of previouslyrecorded downward reflecting energies is one of the unique processesthat compute internal multiples. At the same time, upward reflectingenergies 1347 and 1348 (f and g in FIG. 13) are computed at eachreflection boundary 1022 and 1023, and saved.

At the surface, the wavefield is zeroed. During the second downwardpropagation, the upward reflecting energies 1347 and 1348 (f and g inFIG. 13) are recorded and the previously recorded downward reflections1322 and 1323 (e′ and d′) are added to the downward propagatingwavefield.

Step 4: The second upward propagation begins with zero energy at thebottom 1024 and accumulates the upward reflecting energies 1436 and 1437(g′ and f′ in FIG. 14) that were computed and recorded during the seconddownward propagation (Step 3). The upward reflections contain energyaccumulated only from the downward reflections 1322 and 1323 (e′ and d′)recorded during the first upward propagation (Step 2), and therefore thewavefield recorded at the surface receiver position R contains onlyinternal multiples (FIG. 14).

For the second upward propagation, the wavefield energy at the bottom1024 is again zeroed, and previously recorded upward reflections 1347and 1348 (f and g) are added to the upward propagating wavefield 1436and 1437 (f and g′). The wavefield recorded at the surface receiverlocation (R) contains only first order internal multiples.

Using a simple four-layered earth model, one of the embodiments of thecurrent invention is illustrated above, where only a first orderinternal multiple is predicted. If the model has more or less layers,then the steps may need to be adjusted accordingly without departingfrom the essence of the embodiment. Once the internal multiples arepredicted, they can be removed during further data processing andproduce multiple-free seismic data. Processed seismic data is used toidentify and locate subsurface geological structures, such as reservoirsof hydrocarbon, water or other valuable materials.

Optional Step—Higher Order Multiples

If the prediction of higher order multiples is desired, the samefour-step process can be used. For example, if three propagations arerun, downward reflections are computed and recorded during the secondupward propagation (Step 4 and FIG. 14) just as they were during thefirst upward propagation (Step 2 and FIG. 12), then the thirdpropagation would proceed just as described for the second propagation(FIG. 15). In this manner, any order of internal multiples can becomputed in a single job by propagating the wavefield the requirednumber of times. All propagations work the same way, except for thefirst downward propagation, which does not accumulate any downwardreflecting energy, and the last upward propagation, which does notcompute any downward reflections (FIG. 15). With additional computation,any higher order multiples can be calculated.

Optional Step—Surface Multiples

If the wavefield is not zeroed at surface, both surface multiples andinternal multiples are computed. Surface multiples can be computed inaddition to internal multiples if the downward propagations (Step 3)begin with the planar wavefield recorded from the previous upwardpropagation (Step 2, see FIG. 16). If two propagations are run, thismethod computes first order surface and first order internal multiples.If N>2 propagations are run, this method computes from 1 to N−1 ordersurface multiples and from 1 to N−1 order internal multiples, andcombinations of surface plus internal multiples, where the order of thesurface multiples (I) and the order of the internal multiples (J) isI+J=N−1.

Recorded Shot as Initial Wavefield

WEM IMP does not require any input data, and the first propagationbegins at a single point source. In this case, N propagations produceinternal multiples of order 1 to N−1.

However, the primary wavefield (the wavefield resulting from the firstdown/up propagation from a point source) can be replaced with therecorded shot data.

The recorded shot contains primaries and all recorded multiples,including all orders of surface and internal multiples. When shots areinput, two down/up propagations may be required to compute first ordermultiples. This is because the first propagation is needed to computethe downward reflections that are accumulated in the second propagation.Therefore, this method still requires N propagations to compute internalmultiples of orders 1 to N−1.

After the two down/up propagations, all events recorded by the shot havea surface multiple plus all possible first order internal multiple addedto them. FIG. 17 shows a primary 1712 (a), a first order internalmultiple 1714 (b), and a first order surface multiple 1716 (c). As shownin FIG. 17, WEM IMP is used to predict first order internal multiples(1722, 1724 and 1726). In this example, the implemented algorithmaccording to a method of the invention: 1) converts the primary to afirst order surface multiple plus a first order internal multiple, 2)converts the first order internal multiple to a first order surfacemultiple plus a second order internal multiple, and 3) converts thefirst order surface multiple to a second order surface multiple plus afirst order internal multiple. This is a more efficient way to predicthigher order internal plus surface multiples.

FIG. 18 illustrates a flowchart 1800 of one implementation of anembodiment. The process starts at the beginning step 1802. At step 1804,it checks whether the propagation is a first or a subsequentpropagation. If it is the very first propagation, then it needs to beinitialized at 1806. The wavefield as point source located at the sourceposition is initialized with energy of 1. For subsequent propagation,the wavefield at top as 2D plane is initialized with energy at 0 in step1808. At propagation step 1812, downward propagate the wavefield to nextdeeper depth step, compute and preserve information about any upwardreflection energy. At step 1814, it is checked whether a bottom isreached. If not, the propagation step 1812 is repeated, until the bottomis reached. Once the bottom is reached, at step 1816, the wavefield isinitialized as 2D plane with energy at 0.

At step 1824, it is checked whether the last propagation is reached. Ifnot, then at step 1828, upward propagation is performed. The 2D planewavefield is propagated upward to the next shallower depth step; computeand preserve information about any downward reflecting energy; andaccumulate upward reflecting energy preserved in step 1806 or 1808. Iflast propagation has reached, then at step 1826, upward propagate 2Dplane wavefield to next shallower depth step and accumulate upwardreflecting energy preserved in step 1806 or 1808. No downward reflectioncomputation is done here.

At step 1834, it is checked whether the propagation reaches the surfacewhere the receiver is located. If no, then step 1828 or 1826 is repeatedto upward propagate the wavefield to the surface. If yes, then at step1836 save 2D plane wavefield at the surface.

At step 1844, it is checked whether the last propagation is reached. Ifnot, then repeat propagation starting from step 1804. If yes, then it isdone and stops at step 1850. The receiver now has the energies from alldesired propagation paths.

Once the energies from various multiples are predicted from the aboveprocess, they can be eliminated from the seismic data during the dataprocessing. After the data processing, where other data processing stepsmay also be performed to improve the data quality in various otheraspects, multiple-free seismic data can be obtained. Using the improveddata, geophysicists can find geological abnormalities in the subsurfacestructures and find possible locations for various targets they arelooking for, such as hydrocarbon deposits. Based on the finding, furtheractions may be taken, such as drilling additional wells at thehydrocarbon deposits, changing drilling trajectories to avoid hazardoussubsurface structures or increasing oil production etc.

FIG. 19 illustrates a computing system 1900, into which implementationsof various techniques described herein may be implemented. The computingsystem 1900 may include one or more system computers 1930, which may beimplemented as any conventional personal computer or server. However,those skilled in the art will appreciate that implementations of varioustechniques described herein may be practiced in other computer systemconfigurations, including hypertext transfer protocol (HTTP) servers,hand-held devices, multiprocessor systems, microprocessor-based orprogrammable consumer electronics, network PCs, minicomputers, mainframecomputers, and the like.

The system computer 1930 may be in communication with disk storagedevices 1929, 1931, and 1933, which may be external hard disk storagedevices. It is contemplated that disk storage devices 1929, 1931, and1933 are conventional hard disk drives, and as such, will be implementedby way of a local area network or by remote access. Of course, whiledisk storage devices 1929, 1931, and 1933 are illustrated as separatedevices, a single disk storage device may be used to store any and allof the program instructions, measurement data, and results as desired.

In one implementation, seismic data from the receivers may be stored indisk storage device 1931. The system computer 1930 may retrieve theappropriate data from the disk storage device 1931 to process seismicdata according to program instructions that correspond toimplementations of various techniques described herein. The programinstructions may be written in a computer programming language, such asC++, Java and the like. The program instructions may be stored in acomputer-readable medium, such as program disk storage device 1933. Suchcomputer-readable media may include computer storage media andcommunication media. Computer storage media may include volatile andnon-volatile, and removable and non-removable media implemented in anymethod or technology for storage of information, such ascomputer-readable instructions, data structures, program modules orother data. Computer storage media may further include RAM, ROM,erasable programmable read-only memory (EPROM), electrically erasableprogrammable read-only memory (EEPROM), flash memory or other solidstate memory technology, CD-ROM, digital versatile disks (DVD), or otheroptical storage, magnetic cassettes, magnetic tape, magnetic diskstorage or other magnetic storage devices, or any other medium which canbe used to store the desired information and which can be accessed bythe system computer 1930. Combinations of any of the above may also beincluded within the scope of computer readable media.

In one implementation, the system computer 1930 may present outputprimarily onto graphics display 1927, or alternatively via printer 1928.The system computer 1930 may store the results of the methods describedabove on disk storage 1929, for later use and further analysis. Thekeyboard 19219 and the pointing device (e.g., a mouse, trackball, or thelike) 1925 may be provided with the system computer 1930 to enableinteractive operation.

The system computer 1930 may be located at a data center remote from thesurvey region. The system computer 1930 may be in communication with thereceivers (either directly or via a recording unit, not shown), toreceive signals indicative of the reflected seismic energy. Thesesignals, after conventional formatting and other initial processing, maybe stored by the system computer 1930 as digital data in the diskstorage 1931 for subsequent retrieval and processing in the mannerdescribed above. While FIG. 19 illustrates the disk storage 1931 asdirectly connected to the system computer 1930, it is also contemplatedthat the disk storage device 1931 may be accessible through a local areanetwork or by remote access. Furthermore, while disk storage devices1929, 1931 are illustrated as separate devices for storing input seismicdata and analysis results, the disk storage devices 1929, 1931 may beimplemented within a single disk drive (either together with orseparately from program disk storage device 1933), or in any otherconventional manner as will be fully understood by one of skill in theart having reference to this specification.

While the foregoing is directed to implementations of various techniquesdescribed herein, other and further implementations may be devisedwithout departing from the basic scope thereof, which may be determinedby the claims that follow. Although the subject matter has beendescribed in language specific to structural features and/ormethodological acts, it is to be understood that the subject matterdefined in the appended claims is not necessarily limited to thespecific features or acts described above. Rather, the specific featuresand acts described above are disclosed as example forms of implementingthe claims.

1. A computer implemented method for geophysical survey where multiplereflections in seismic surveys are attenuated by predicting suchmultiples using a multi-layer earth model, the method comprising: (a)propagating a wavefield downward until reaching a bottom of the earthmodel, computing and preserving information about any upward reflectingenergies at each layer boundary; (b) propagating the wavefield upwarduntil reaching the surface, computing and preserving information aboutany downward reflecting energies at each layer boundary; and (c)repeating step (a) and (b) at least once and accumulating upwardenergies towards a receiver as a prediction of multiples.
 2. The methodof claim 1, wherein in step (a), when the wavefield propagates downwardand reaches the bottom, the upward reflecting energy is set to zero;wherein in step (b), when the wavefield propagates upward and reachesthe surface, the downward reflecting energy is set to zero; wherein thepredicted multiples include internal multiples.
 3. The method of claim1, wherein in step (a), when the wavefield propagates downward andreaches the bottom, the upward reflecting energy is set to zero; whereinin step (b), when the wavefield propagates upward and reaches thesurface, the downward reflecting energy is set to non-zero; and whereinthe predicted multiples include internal multiples and surfacemultiples.
 4. The method of claim 1, further comprising: repeating step(a) and (b) N times and accumulating upward energies towards a receiveras a prediction of multiples of (N+1)'th order, wherein N=I+J, where Iis the order of surface multiples and J is the order of internalmultiples.
 5. The method of claim 1, wherein the initial data forpropagating is recorded shot data at point sources.
 6. The method ofclaim 1, wherein the initial data for propagating wavefield is notrequired.
 7. The method of claim 1, further comprising identifying ageological structure.
 8. A computer-readable medium having storedthereon computer-executable instructions which, when executed by acomputer, cause the computer to perform the following steps: (a)propagating a wavefield downward until reaching a bottom of the earthmodel, computing and preserving information about any upward reflectingenergies at each layer boundary; (b) propagating the wavefield upwarduntil reaching the surface, computing and preserving information aboutany downward reflecting energies at each layer boundary; and (c)repeating step (a) and (b) at least once and accumulating upwardenergies towards a receiver as a prediction of multiples.
 9. Thecomputer-readable medium of claim 8, wherein in step (a), when thewavefield propagates downward and reaches the bottom, the upwardreflecting energy is set to zero; wherein in step (b), when thewavefield propagates upward and reaches the surface, the downwardreflecting energy is set to zero; Wherein the predicted multiplesinclude internal multiples.
 10. The computer-readable medium of claim 8,wherein in step (a), when the wavefield propagates downward and reachesthe bottom, the upward reflecting energy is set to zero; wherein in step(b), when the wavefield propagates upward and reaches the surface, thedownward reflecting energy is set to non-zero; and wherein the predictedmultiples include internal multiples and surface multiples.
 11. Thecomputer-readable medium of claim 8, further comprising: repeating step(a) and (b) N times and accumulating upward energies towards a receiveras a prediction of multiples of (N+1)'th order, wherein N=I+J, where Iis the order of surface multiples and J is the order of internalmultiples.
 12. The computer-readable medium of claim 8, wherein theinitial data for propagating is recorded shot data at point sources. 13.The computer-readable medium of claim 8, wherein the initial data forpropagating wavefield is not required.
 14. A seismic data processingsystem comprising: a processor; and a storage medium having storedthereon computer-executable instructions which, when executed by theprocessor, cause the system to perform the following steps: (a)propagating a wavefield downward until reaching a bottom of the earthmodel, computing and preserving information about any upward reflectingenergies at each layer boundary; (b) propagating the wavefield upwarduntil reaching the surface, computing and preserving information aboutany downward reflecting energies at each layer boundary; and (c)repeating step (a) and (b) at least once and accumulating upwardenergies towards a receiver as a prediction of multiples.
 15. The systemof claim 14, wherein in step (a), when the wavefield propagates downwardand reaches the bottom, the upward reflecting energy is set to zero;wherein in step (b), when the wavefield propagates upward and reachesthe surface, the downward reflecting energy is set to zero; wherein thepredicted multiples include internal multiples.
 16. The system of claim14, wherein in step (a), when the wavefield propagates downward andreaches the bottom, the upward reflecting energy is set to zero; whereinin step (b), when the wavefield propagates upward and reaches thesurface, the downward reflecting energy is set to non-zero; and whereinthe predicted multiples include internal multiples and surfacemultiples.
 17. The system of claim 14, where in the steps furthercomprising: repeating step (a) and (b) N times and accumulating upwardenergies towards a receiver as a prediction of multiples of (N+1)'thorder, wherein N=I+J, where I is the order of surface multiples and J isthe order of internal multiples.
 18. The system of claim 14, wherein theinitial data for propagating is recorded shot data at point sources. 19.The system of claim 14, wherein the initial data for propagatingwavefield is not required.